This invention relates to sealed enclosures in which electronic components are housed, and more particularly, to apparatus for controlling the temperature within the enclosure to protect the components regardless of the temperature extremes to which the enclosure is subjected.
Certain electronic equipment, for example, traffic monitoring equipment, includes sensors and associated electronic processing circuitry housed in a sealed enclosure. To perform the traffic monitoring function, the enclosure is typically installed outdoors atop a pole or similar support. As a result, the enclosure and the equipment housed in it may be required to operate at temperatures ranging between -40.degree. C. to +50.degree. C. (-40.degree. F. to +122.degree. F.). At very high temperatures, ambient heat must be extracted from the enclosure; while in extremely cold conditions, heat must be supplied to the enclosure.
A traditional approach to thermal management in electrical and electronic equipment is use of heat sinks with components which cannot be allowed to get too hot. Heat sinks are used because their surface area is larger than that of the electronic components mounted on a heat sink and this helps to readily dissipate heat generated by the components. Heat transfer is often enhanced by forcing air flow over the heat sink, typically by use of a fan. The warm air drawn away from the components is then vented outside the enclosure in which the components are housed, typically through openings in the enclosure.
Another approach to thermal management involves use of liquids and thermoelectric cooling for heat extraction. Liquid cooling requires a working fluid, typically chilled water, and a pump to move the fluid from a source through a piping arrangement for heat produced by the components to be drawn to the fluid as it flows past the components. Thermoelectric cooling requires an electrical supply to operate. Again, forced air flow over the components may be used to increase heat extraction, and this requires openings in the enclosure so the heated air can be discharged to the outside environment. With heat sinks, liquid cooling, and thermoelectric cooling, the heat transfer is not 100% efficient, so a certain amount of heat remains inside the enclosure.
When electronic equipment is housed in a sealed enclosure, the above heat extraction approaches are not effective because air within the enclosure is not exchanged with the outside environment. Resulting heat build-up produces a temperature rise within the enclosure which is detrimental to the electronics. A means is therefore required to transfer heat from the equipment to the outside of the enclosure. In certain electronic applications in which components are housed in a sealed enclosure, a printed circuit board is used which has a copper inner-layer. Electronic components requiring heat extraction contact this copper layer. Edges of the board contact sides of the enclosure. While effective, this approach is very expensive. Further, this approach does not have the capability of disabling the heat extraction mechanism when the equipment is operating in a cold environment and heat needs to be retained within the enclosure to maintain the equipment within a desired band of operating temperatures.